Bridge tap
Encyclopedia
Bridged tap or bridge tap is a long-used method of cabling for telephone lines. One cable pair (of wires) will "appear" in several different terminal locations (poles or pedestal
s). This allows the telephone company to use or "assign
" that pair to any subscriber near those terminal locations. Once that customer disconnects, that pair becomes usable at any of the terminals. In the days of party line
s, 2, 4, 6, or 8 users were commonly connected on the same pair which appeared at several different locations.
A bridge tap has no hybrid coil or other impedance matching
components, just a “T” (or branch) in the cable. Thus the bridge presents an impedance mismatch. The unused branch of the T is usually left with no device connected to its end, thus has no electrical termination
. Both the tap and its unterminated branch cause unwanted signal reflection
s, also called echoes.
Digital subscriber line
s (DSL) can be affected by a bridged tap, depending on where the tap is bridged. DSL signals reflect from the discontinuities, sending the signal back through the cable pair, much like a tennis ball against a brick wall. The echoed signal is now out of phase and mixed with the original, creating, among other impairments, attenuation distortion
. The modem receives both signals, gets confused and "takes errors" or cannot sync
. If the bridged tap is long, the signal bounces back only in very attenuated
form. Therefore, the modem will ignore the weaker signal and show no problem.
A bridge tap can also be referred to as a 'multiple' or a telephone pair 'in multiple'. Newton's Telecom Dictionary, 23rd Edition; 2003; Newton, Harry; p110
Pedestal
Pedestal is a term generally applied to the support of a statue or a vase....
s). This allows the telephone company to use or "assign
Provisioning
In telecommunication, provisioning is the process of preparing and equipping a network to allow it to provide services to its users. In NS/EP telecommunications services, "provisioning" equates to "initiation" and includes altering the state of an existing priority service or capability.In a...
" that pair to any subscriber near those terminal locations. Once that customer disconnects, that pair becomes usable at any of the terminals. In the days of party line
Party line (telephony)
In twentieth-century telephone systems, a party line is an arrangement in which two or more customers are connected directly to the same local loop. Prior to World War II in the United States, party lines were the primary way residential subscribers acquired local telephone service...
s, 2, 4, 6, or 8 users were commonly connected on the same pair which appeared at several different locations.
A bridge tap has no hybrid coil or other impedance matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
components, just a “T” (or branch) in the cable. Thus the bridge presents an impedance mismatch. The unused branch of the T is usually left with no device connected to its end, thus has no electrical termination
Electrical termination
Electrical termination of a signal involves providing a terminator at the end of a wire or cable to prevent an RF signal from being reflected back from the end, causing interference...
. Both the tap and its unterminated branch cause unwanted signal reflection
Signal reflection
Signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber, some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the...
s, also called echoes.
Digital subscriber line
Digital Subscriber Line
Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
s (DSL) can be affected by a bridged tap, depending on where the tap is bridged. DSL signals reflect from the discontinuities, sending the signal back through the cable pair, much like a tennis ball against a brick wall. The echoed signal is now out of phase and mixed with the original, creating, among other impairments, attenuation distortion
Attenuation Distortion
Attenuation distortion is the distortion of an analog signal that occurs during transmission when the transmission medium does not have a flat frequency response across the bandwidth of the medium or the frequency spectrum of the signal....
. The modem receives both signals, gets confused and "takes errors" or cannot sync
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
. If the bridged tap is long, the signal bounces back only in very attenuated
Attenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
form. Therefore, the modem will ignore the weaker signal and show no problem.
A bridge tap can also be referred to as a 'multiple' or a telephone pair 'in multiple'. Newton's Telecom Dictionary, 23rd Edition; 2003; Newton, Harry; p110